The present invention is concerned with a bioreactor for the biochemical treatment of liquids containing organic matter. The invention is more particularly directed to a bioreactor suitable for the fermentation of aqueous solutions of fermentable sugars, such as bisulfite liquors originating from the pulp and paper industry, to produce ethanol as a valuable by-product.
The need to treat on a profitable basis the bisulfite liquors from the pulp and paper industry is becoming more and more urgent due to the pollution regulations imposed by governments.
The annual world production of pulp by the sulfite process is about 11.5 million metric tons on dry basis, 1.5 millions of which originate from the United States and 2 millions from Canada. For each ton of dry sulfite pulp produced, there is about one ton of waste products in the form of solids dissolved in water, a major portion of this bisulfite liquor being dumped into rivers. Such a liquor has the following typical composition:
______________________________________ % Total Solids ______________________________________ Lignosulfonate 52 Extractive matters 3 Poly and oligosaccharides 6 Monosaccharides 23 galactose: 3 glucose: 3 mannose: 11 arabinose: 1 xylose: 5 Glucuronic acid 1 Aldonic acid 4 Sulfonated sugar 3 Acetic acid 2 Methanol 1 Calcium bisulfite 5 100% ______________________________________
The production of ethanol by fermentation of the sugars contained in the above liquor is of particular interest since the ethanol can be readily separated from the remainder of the liquid after fermentation. The conversion rate of biochemical reactions, however, is much more slow compared to the conversion rates of pyrolysis or direct combustion reactions. Thus, on an industrial scale, the use of a biochemical process necessitates a bioreactor having a high productivity.
The productivity of conventional stirred tank fermentors operated either continuously or discontinuously is limited by the specific growth rate of the microorganism cells. In a continuously operated tank-type reactor, the substrate circulates continuously through the reactor. When the medium is perfectly agitated, the cell concentration is the same everywhere in the reactor and the cells thus flow out of the reactor together with the substrate at the same concentration as in the reactor. An increase in the flow rate will therefore dilute the cells in the reactor, resulting in a lowering of the reactor productivity since the rate of product formation is proportional to the number of microorganism cells in the system. Thus, at a sufficiently high flow rate, the dilution rate will exceed the specific growth rate of the cells corresponding to the operating substrate concentration, causing the so-called phenomenon of cell washout.
On the other hand, various tubular bioreactors have been proposed, such as the free-cell reactors with or without cell recycle and the immobilized-cell reactors. In the free-cell reactor without cell recycle, the substrate and microorganism cells are introduced at the same time into the reactor, that is, at the beginning. Once the reactor is filled, the substrate circulates through the reactor. The productivity of this type of reactor is also affected by the same phenomenon of cell washout as in the continuously operated stirred tank-type reactor. When such a free-cell reactor is operated with cell recycle, it requires the additional use of a centrifugal machine in order to accomplish the cell recycle. This is not only expensive both in terms of capital investment and subsequent operating costs, but could also cause destruction of the cells. Moreover, it has been observed that the specific rate of product formation drops considerably as soon as the substrate concentration falls below 10 g/l.
By immobilizing the cells inside the reactor, using for instance a packing of gelatin coated ceramic particles treated with glutaraldehyde, the reactor can operate at a dilution rate exceeding the specific growth rate of the microorganism cells. Although such an immobilized-cell reactor has a productivity which is considerably higher than that of a free-cell reactor without cell recycle, it suffers from several disadvantages. Firstly, the film of gelatin which coats the ceramic particles swells during operation, causing a reduction of the nominal void percentage of the packing to about 50-55%. In other words, the usable volume of the reactor is only about 50% of the total real volume. The immobilization step is also very delicate and time consuming and requires additional equipment. The glutaraldehyde which serves to bind the cells to the gelatin molecules further acts as a bactericide; thus, too much glutaraldehyde could kill the immobilized cells whereas too little would allow the gelatin to dissolve in the flowing liquid to be fermented and thus cause a rupture of the bonds by means of which the cells are attached to the ceramic particles. The time required to start the reactor, after having filled the latter with the packing, disinfected the whole and inserted the cell culture, is about 8 hours; however, one must also consider the time required to recycle the packing between two cycles of operation, which is about 48 hours, Moreover, in the case of contamination, the packing including the gelatin film must be completely regenerated. Finally, there is a progressive dissolution of the gelatin film, which reduces the quantity of immobilized cells, thus resulting in a continuous lowering of the reactor productivity as a function of the operating time.